This is story related and cannot be missed. You get at at the completion of Eagle Peak area which contains two main tasks.
First one involves the volcano
*** Spoiler - click to reveal ***
Lots of little, exploding lava spiders, bring a large capacity gun, and one very large lava spider boss. I also highly recommend it have Life Leach.
The boss has phases, mostly he leaps at you early on, look for the ground targets and move it. When he climbs the wall just run and don’t shoot, he’s pretty damage resistant at that point and you do not want to get hit by those ground targets.
I didn’t notice any real change in phase 2 other than he spawns spiders and farts lava (I hope it’s lava). The lava fart has an obvious ground target, it’s safer to be closer to him during this phase.
Phase three he’s basically a giant worm/tree trunk with the spider as a head. I just circle strafed this phase and used movement abilities when I couldn’t spray down his spider minions. Honestly this was the easiest phase, he sprays a fan of lava and rotates but as long as you don’t aim you can run and shoot and keep ahead of it, and he’s a very large target so you should easily hit him even with the poor accuracy.
and getting to the radio tower cable car
*** Spoiler - click to reveal ***
This is a regular slog of human enemies though you’ll run into a new enemy type called a Spotter, he throws a targeting grenade that rains down artillery after a few seconds. Needless to say, do NOT stand there.
The terrain is really on the enemies’ side here in several areas, you can shoot away a lot of the wooden covers with a shotgun if that helps but mostly keep moving. Charging forward will spawn multiple Breachers so careful when you do that in most areas. There are a few waves of the melee guys in most areas too.
You should be getting your first purple rewards just now, snagging a good gun with life leach and a large capacity will help.
The achievement pops during the ending cut scene after sending the cable car back down to the base camp.